As the technical development of and the demand for mobile devices have increased, the demand for secondary batteries as an energy source has rapidly increased. Among the secondary batteries, lithium secondary batteries, which have high energy density and discharge voltage, have been carried out much research and commercially available and widely used.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries which are thin enough to be applied to products, such as mobile phones, is very high. However, in terms of the material for batteries, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, exhibiting high energy density, discharge voltage, and output stability is very high.
The secondary battery according to its shape can be classified into a cylinder type battery cell, a prismatic type battery cell, and a pouch type battery cell. Among them, the cylinder type secondary battery comprises an electrode assembly, a cylinder type can including the electrode assemble, and a can assembly combined to the top of the can.
In the secondary battery, an electrode assembly installed in the battery case is an electricity generating element, which is composed of stacked structure of a positive electrode, a separator, and a negative electrode and can be charge and discharge. The electrode assembly can be approximately classified into a jelly-roll type, which wound up by interposed a separator between a positive electrode and a negative electrode of a sheet type applied an electrode active material slurry, a stacked type sequentially laminated in the state of a separator interposed between a plurality of positive electrode and negative electrode, and a stacked/folding type wound with unit cells of stacked type to long separated film. Among them, the jelly-roll type electrode assembly has advantages such as it is easy to manufacture and has high energy destiny per unit weight, thereby it is widely used.
The jelly-roll type electrode assembly composed of jelly-roll type wound with a positive electrode, a negative electrode, and a separator interposed between the two electrodes as cylinder type, and a positive electrode tab and a negative electrode tab each drawn from a positive electrode and a negative electrode. Generally, a positive electrode tab drawn to the upper part, and a negative electrode drawn to the lower part.
A can is a container of metal material having nearly cylinder form in cylinder type secondary batteries, and formed by manufacturing method such as deep drawing. Therefore, the can itself can be a terminal.
A cap assembly has a structure in which an upper end cap forming a positive electrode terminal, a PTC element configured to greatly increase a resistance of a battery to cut off a current when temperature inside the battery rises, a safety vent configured to cut off the current or exhaust gas when a pressure inside the battery rises, a gasket configured to electrically isolate the safety vent from a cap plate except for certain portions, and the cap plate to which a positive electrode terminal connected to the positive electrode is connected are sequentially stacked.
The positive electrode of the electrode assembly is electrically connected to one component of the cap assembly through the positive electrode tab which is drawn upward from the positive electrode, and the negative electrode is connected to a bottom surface of the can through the negative electrode tab which is drawn downward from the negative electrode. Of course, the electrode may be designed by changing polarity.
Further, an upper insulation member for insulating between the electrode assembly and the cap assembly is located therebetween, and a lower insulation member for insulating between the electrode assembly and the bottom surface of the can is located therebetween.
Meanwhile, a lithium secondary battery as an electrode active material comprises a positive electrode including lithium transition metal oxide, a negative electrode including a carbon-based active material, and a porous separator. The positive electrode is prepared by coating a positive electrode slurry to an aluminum foil, and the negative electrode is prepared by coating a negative electrode slurry including a carbon-based active material to a copper foil.
The positive electrode slurry and the negative electrode slurry added a conductive material to improve electrical conductivity of an active material. Especially, a lithium transition metal oxide used as a positive active material fundamentally has low electrical conductivity, thereby the positive electrode slurry requisitely added a conductive material.
As the conductive material, carbon-based materials such as graphite such as natural graphite and artificial graphite, carbon black such as carbon black, acetylene black, channel black, furnace black, lamp black, summer black, and the like are mainly used, and in some cases, conductive fibers such as carbon fibers, metal fibers, and the like are used. Specific examples of commercially available conductive materials may include various acetylene black products (available from Chevron Chemical Company), Denka Black (available from Denka Singapore Private Limited), various products available from Gulf Oil Company, Vulcan XC-72 (available from Cabot Company), Super P (Timcal Co., Ltd.), and the like.
As demand for electricity is increasing, batteries are gradually having high capacity and a high loading electrode having high energy density has appeared. In the case of a high loading electrode, compared to a low loading electrode, cycle performance is low as lithium ion delivery to foil in an electrode is very low.
Specifically, in order to increase linearity of lithium ion in a high loading electrode, methods such as increasing porosity in an electrode or mechanically preparing pores in an electrode after manufacturing an electrode have been suggested.
However, in the case of increasing porosity in an electrode, because of increasing in thickness, energy density is decreased, and in the case of preparing pores in an electrode after manufacturing an electrode, local electrode degradation can be happened because of decrease in an electrode active material and artificial form.